Method and apparatus for transmitting and receiving signal in inter-vehicle communication system

ABSTRACT

The present specification relates to a method for transmitting and receiving a signal to/from a base station by a terminal in an inter-vehicle communication system, wherein the method for transmitting a signal may comprise: a step for receiving a reference signal; a step for feeding-back channel information on the basis of the received reference signal; and a step for receiving data on the basis of the channel information, and wherein the terminal includes a plurality of distributed antenna units (DUs), and if selection of whether each of the plurality of DUs is activated is possible, the channel information may include DU index set information and channel state information (CSI).

This application is a National Stage Application of InternationalApplication No. PCT/KR2016/000854, filed on Jan. 27, 2016, which claimsthe benefit of U.S. Provisional Application No. 62/167,894, filed on May29, 2015, all of which are hereby incorporated by reference in theirentirety for all purposes as if fully set forth herein.

TECHNICAL FIELD

The present relates to a wireless communication system and, moreparticularly, to a wireless communication system applied to vehicularcommunication. At this time, it is possible to provide a method offeeding back channel information at a vehicle (or user equipment (UE))in a vehicular communication system.

BACKGROUND ART

A brief description will be given of a 3rd Generation PartnershipProject Long Term Evolution (3GPP LTE) system as an example of awireless communication system to which the present invention can beapplied.

FIG. 1 illustrates a configuration of an Evolved Universal MobileTelecommunications System (E-UMTS) network as an exemplary wirelesscommunication system. The E-UMTS system is an evolution of the legacyUMTS system and the 3GPP is working on the basics of E-UMTSstandardization. E-UMTS is also called an LTE system. For details of thetechnical specifications of UMTS and E-UMTS, refer to Release 7 andRelease 8 of “3rd Generation Partnership Project; TechnicalSpecification Group Radio Access Network”, respectively.

Referring to FIG. 1, the E-UMTS system includes a User Equipment (UE),an evolved Node B (eNode B or eNB), and an Access Gateway (AG) which islocated at an end of an Evolved UMTS Terrestrial Radio Access Network(E-UTRAN) and connected to an external network. The eNB may transmitmultiple data streams simultaneously, for broadcast service, multicastservice, and/or unicast service.

A single eNB manages one or more cells. A cell is set to operate in oneof the bandwidths of 1.25, 2.5, 5, 10, 15 and 20 Mhz and providesDownlink (DL) or Uplink (UL) transmission service to a plurality of UEsin the bandwidth. Different cells may be configured so as to providedifferent bandwidths. An eNB controls data transmission and reception toand from a plurality of UEs. Regarding DL data, the eNB notifies aparticular UE of a time-frequency area in which the DL data is supposedto be transmitted, a coding scheme, a data size, Hybrid Automatic RepeatreQuest (HARQ) information, etc. by transmitting DL schedulinginformation to the UE. Regarding UL data, the eNB notifies a particularUE of a time-frequency area in which the UE can transmit data, a codingscheme, a data size, HARQ information, etc. by transmitting ULscheduling information to the UE. An interface for transmitting usertraffic or control traffic may be defined between eNBs. A Core Network(CN) may include an AG and a network node for user registration of UEs.The AG manages the mobility of UEs on a Tracking Area (TA) basis. A TAincludes a plurality of cells.

While the development stage of wireless communication technology hasreached LTE based on Wideband Code Division Multiple Access (WCDMA), thedemands and expectation of users and service providers are increasing.Considering that other radio access technologies are under development,a new technological evolution is required to achieve futurecompetitiveness. Specifically, cost reduction per bit, increased serviceavailability, flexible use of frequency bands, a simplified structure,an open interface, appropriate power consumption of UEs, etc. arerequired.

In addition, recently, interest in a vehicular communication system hasincreased. More specifically, interest in a method of applying wirelesscommunication technology and position tracking technology (globalpositioning system (GPS)) to a vehicle and providing a service such asvehicle diagnosis, theft detection, route guidance or traffic serviceprovision to a driver who uses the vehicle in real time has increased.At this time, there is a need for methods of efficiently performingvehicular communication in consideration of the appearance and mobilityof a vehicle and interference with another vehicle in a vehicularcommunication system.

DISCLOSURE Technical Problem

An object of the present invention is to provide a method of feedingback channel information in a vehicular communication system, and anapparatus therefor.

Another object of the present invention is to provide an efficientchannel information feedback method for vehicular communication inconsideration of an environment in which vehicles are denselydistributed.

Technical Solution

The object of the present invention can be achieved by providing amethod of transmitting and receiving a signal from a user equipment (UE)to a base station, the method comprising: receiving a reference signal;feeding back channel information based on the received reference signal;receiving data based on the channel information, wherein the UE includesa plurality of distributed antenna units (DUs), and wherein the channelinformation includes DU index set information and channel stateinformation (CSI) when activation of each of the plurality of DUs isselectable.

In another aspect of the present invention, provided herein is a userequipment (UE) device for transmitting and receiving a signal, the UEcomprising: a reception module configured to receive information from anexternal device; a transmission module configured to transmitinformation to an external device; and a processor configured to controlthe reception module and the transmission module, wherein the processoris further configured to: receives a reference signal using thereception module; feeds back channel information based on the receivedreference signal using the transmission module; and receives data basedon the channel information using the reception module, and wherein theUE includes a plurality of distributed antenna units (DUs), and whereinthe channel information includes DU index set information and channelstate information (CSI) when activation of each of the plurality of DUsis selectable.

In addition, the following matters are commonly applicable to the methodof transmitting and receiving a signal from the UE to the base stationin the wireless communication system.

In one embodiment of the present invention, the DU index set informationincludes DU index information set based on activation of each of theplurality of DUs. The DU index information is set based on the number ofDUs included in the UE and locations of the DUs.

In one embodiment of the present invention, the index set informationincludes 2{circumflex over ( )}N pieces of DU index information when thenumber of DUs included in the UE is N.

In one embodiment of the present invention, the DU index set informationincludes only the DU index information selectable by the number ofactivated DUs when the number of activated DUs among the DUs included inthe UE is determined.

In one embodiment of the present invention, the data is received by thebase station based on the DU index set included in the channelinformation and effective channel information acquired through thechannel state information.

In one embodiment of the present invention, each of the DUs included inthe UE includes a plurality of antenna ports.

In one embodiment of the present invention, the DU index set informationincludes information on a mapping relationship between the plurality ofantenna ports and the plurality of DUs.

In one embodiment of the present invention, further comprising feedingback DU activation information. The DU activation information includesthe number of DUs capable of being simultaneously activated anddeactivated and combination information based on a radio frequency (RF)structure.

In one embodiment of the present invention, the first type of UE feedsthe channel information back to the base station and receives DUselection control information from the base station when the UE is afirst type of UE.

In one embodiment of the present invention, the second type of UE feedsthe channel information back to neighboring UEs and receives channelinformation from the neighboring UEs to control DU selection when the UEis a second type of UE.

In one embodiment of the present invention, the first type of UE and thesecond type of UE coexist in one vehicular communication system.

Advantageous Effects

According to the present invention, it is possible to provide a methodof feeding back channel information in a vehicular communication systemand an apparatus therefor.

According to the present invention, it is possible to provide anefficient channel information feedback method for vehicularcommunication in an environment in which vehicles are denselydistributed.

The effects which can be obtained by the present invention are notlimited to the above-described effects and other effects which are notdescribed herein will become apparent to those skilled in the art fromthe following description.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically illustrating a network structure of anE-UMTS as an exemplary radio communication system according to anembodiment of the present invention.

FIG. 2 is a diagram illustrating structures of a control plane and auser plane of a radio interface protocol between a UE and an E-UTRANbased on the 3GPP radio access network specification according to anembodiment of the present invention.

FIG. 3 is a diagram illustrating physical channels used in a 3GPP systemand a general signal transmission method using the same according to anembodiment of the present invention.

FIG. 4 is a diagram illustrating the structure of a radio frame used inan LTE system according to an embodiment of the present invention.

FIG. 5 is a diagram showing the structure of a downlink radio frame usedin an LTE system according to an embodiment of the present invention.

FIG. 6 is a diagram showing the structure of an uplink radio frame usedin an LTE system according to an embodiment of the present invention.

FIG. 7 is a diagram showing the structure of a general multiple inputmultiple output (MIMO) communication system according to an embodimentof the present invention.

FIG. 8 is a diagram showing a vehicle including a plurality of antennaarrays according to an embodiment of the present invention.

FIG. 9 is a diagram showing a method of selecting a distributed antennaunit (DU) in a state in which a plurality of vehicles is concentrated,according to an embodiment of the present invention.

FIG. 10 is a diagram showing an example of a DU selection combinationaccording to an embodiment of the present invention.

FIG. 11 is a diagram showing a DU including a plurality of antenna portsaccording to an embodiment of the present invention.

FIG. 12 is a diagram showing a method of activating a DU based on an RFstructure according to an embodiment of the present invention.

FIG. 13 is a flowchart illustrating a method of feeding back channelinformation according to an embodiment of the present invention.

FIG. 14 is a block diagram showing a UE apparatus and a base stationapparatus according to an embodiment of the present invention.

BEST MODEL

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description set forth below in connection withthe appended drawings is intended as a description of exemplaryembodiments and is not intended to represent the only embodiments inwhich the concepts explained in these embodiments can be practiced. Thedetailed description includes details for the purpose of providing anunderstanding of the present invention. However, it will be apparent tothose skilled in the art that these teachings may be implemented andpracticed without these specific details.

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered to be optional factors on thecondition that there is no additional remark. If required, theindividual constituent components or characteristics may not be combinedwith other components or characteristics. Also, some constituentcomponents and/or characteristics may be combined to implement theembodiments of the present invention. The order of operations to bedisclosed in the embodiments of the present invention may be changed toanother. Some components or characteristics of any embodiment may alsobe included in other embodiments, or may be replaced with those of theother embodiments as necessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for the convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to another format within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention and theimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

The embodiments of the present invention can be supported by thestandard documents disclosed in any one of wireless access systems, suchas an IEEE 802 system, a 3^(rd) Generation Partnership Project (3GPP)system, 3GPP Long Term Evolution (LTE) and LTE-Advanced (LTE-A) systems,and a 3GPP2 system. That is, the steps or portions, which are notdescribed in order to make the technical spirit of the present inventionclear, may be supported by the above documents. In addition, all theterms disclosed in the present document may be described by the abovestandard documents.

The following technology can be applied to a variety of wireless accesstechnologies, for example, CDMA (Code Division Multiple Access), FDMA(Frequency Division Multiple Access), TDMA (Time Division MultipleAccess), OFDMA (Orthogonal Frequency Division Multiple Access), SC-FDMA(Single Carrier Frequency Division Multiple Access), and the like. CDMAmay be embodied with wireless (or radio) technology such as UTRA(Universal Terrestrial Radio Access) or CDMA2000. TDMA may be embodiedwith wireless (or radio) technology such as GSM (Global System forMobile communications)/GPRS (General Packet Radio Service)/EDGE(Enhanced Data Rates for GSM Evolution). OFDMA may be embodied withwireless (or radio) technology such as Institute of Electrical andElectronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE802-20, and E-UTRA (Evolved UTRA).

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure.

In the entire specification, when a certain portion “includes” a certaincomponent, this indicates that the other components are not excluded,but may be further included unless specially described. The terms“unit”, “-or/er” and “module” described in the specification indicate aunit for processing at least one function or operation, which may beimplemented by hardware, software and a combination thereof.

FIG. 2 is a diagram illustrating structures of a control plane and auser plane of a radio interface protocol between a UE and an E-UTRANbased on the 3GPP radio access network specification according to anembodiment of the present invention.

FIG. 2 illustrates control-plane and user-plane protocol stacks in aradio interface protocol architecture conforming to a 3GPP wirelessaccess network standard between a User Equipment (UE) and an EvolvedUMTS Terrestrial Radio Access Network (E-UTRAN). The control plane is apath in which the UE and the E-UTRAN transmit control messages to managecalls, and the user plane is a path in which data generated from anapplication layer, for example, voice data or Internet packet data istransmitted.

A PHYsical (PHY) layer at Layer 1 (L1) provides information transferservice to its higher layer, a Medium Access Control (MAC) layer. ThePHY layer is connected to the MAC layer via transport channels. Thetransport channels deliver data between the MAC layer and the PHY layer.Data is transmitted on physical channels between the PHY layers of atransmitter and a receiver. The physical channels use time and frequencyas radio resources. Specifically, the physical channels are modulated inOrthogonal Frequency Division Multiple Access (OFDMA) for Downlink (DL)and in Single Carrier Frequency Division Multiple Access (SC-FDMA) forUplink (UL).

The MAC layer at Layer 2 (L2) provides service to its higher layer, aRadio Link Control (RLC) layer via logical channels. The RLC layer at L2supports reliable data transmission. RLC functionality may beimplemented in a function block of the MAC layer. A Packet DataConvergence Protocol (PDCP) layer at L2 performs header compression toreduce the amount of unnecessary control information and thusefficiently transmit Internet Protocol (IP) packets such as IP version 4(IPv4) or IP version 6 (IPv6) packets via an air interface having anarrow bandwidth.

A Radio Resource Control (RRC) layer at the lowest part of Layer 3 (orL3) is defined only on the control plane. The RRC layer controls logicalchannels, transport channels, and physical channels in relation toconfiguration, reconfiguration, and release of radio bearers. A radiobearer refers to a service provided at L2, for data transmission betweenthe UE and the E-UTRAN. For this purpose, the RRC layers of the UE andthe E-UTRAN exchange RRC messages with each other. If an RRC connectionis established between the UE and the E-UTRAN, the UE is in RRCConnected mode and otherwise, the UE is in RRC Idle mode. A Non-AccessStratum (NAS) layer above the RRC layer performs functions includingsession management and mobility management.

DL transport channels used to deliver data from the E-UTRAN to UEsinclude a Broadcast Channel (BCH) carrying system information, a PagingChannel (PCH) carrying a paging message, and a Shared Channel (SCH)carrying user traffic or a control message. DL multicast traffic orcontrol messages or DL broadcast traffic or control messages may betransmitted on a DL SCH or a separately defined DL Multicast Channel(MCH). UL transport channels used to deliver data from a UE to theE-UTRAN include a Random Access Channel (RACH) carrying an initialcontrol message and a UL SCH carrying user traffic or a control message.Logical channels that are defined above transport channels and mapped tothe transport channels include a Broadcast Control Channel (BCCH), aPaging Control Channel (PCCH), a Common Control Channel (CCCH), aMulticast Control Channel (MCCH), a Multicast Traffic Channel (MTCH),etc.

FIG. 3 illustrates physical channels and a general method fortransmitting signals on the physical channels in the 3GPP system.

Referring to FIG. 3, when a UE is powered on or enters a new cell, theUE performs initial cell search (S301). The initial cell search involvesacquisition of synchronization to an eNB. Specifically, the UEsynchronizes its timing to the eNB and acquires a cell Identifier (ID)and other information by receiving a Primary Synchronization Channel(P-SCH) and a Secondary Synchronization Channel (S-SCH) from the eNB.Then the UE may acquire information broadcast in the cell by receiving aPhysical Broadcast Channel (PBCH) from the eNB. During the initial cellsearch, the UE may monitor a DL channel state by receiving a DownLinkReference Signal (DL RS).

After the initial cell search, the UE may acquire detailed systeminformation by receiving a Physical Downlink Control Channel (PDCCH) andreceiving a Physical Downlink Shared Channel (PDSCH) based oninformation included in the PDCCH (S302).

If the UE initially accesses the eNB or has no radio resources forsignal transmission to the eNB, the UE may perform a random accessprocedure with the eNB (S303 to S306). In the random access procedure,the UE may transmit a predetermined sequence as a preamble on a PhysicalRandom Access Channel (PRACH) (S303 and S305) and may receive a responsemessage to the preamble on a PDCCH and a PDSCH associated with the PDCCH(S304 and S306). In the case of a contention-based RACH, the UE mayadditionally perform a contention resolution procedure.

After the above procedure, the UE may receive a PDCCH and/or a PDSCHfrom the eNB (S307) and transmit a Physical Uplink Shared Channel(PUSCH) and/or a Physical Uplink Control Channel (PUCCH) to the eNB(S308), which is a general DL and UL signal transmission procedure.Particularly, the UE receives Downlink Control Information (DCI) on aPDCCH. Herein, the DCI includes control information such as resourceallocation information for the UE. Different DCI formats are definedaccording to different usages of DCI.

Control information that the UE transmits to the eNB on the UL orreceives from the eNB on the DL includes a DL/UL ACKnowledgment/NegativeACKnowledgment (ACK/NACK) signal, a Channel Quality Indicator (CQI), aPrecoding Matrix Index (PMI), a Rank Indicator (RI), etc. In the 3GPPLTE system, the UE may transmit control information such as a CQI, aPMI, an RI, etc. on a PUSCH and/or a PUCCH.

FIG. 4 illustrates a structure of a radio frame used in the LTE system.

Referring to FIG. 4, a radio frame is 10 ms (327200×T_(s)) long anddivided into 10 equal-sized subframes. Each subframe is 1 ms long andfurther divided into two slots. Each time slot is 0.5 ms (15360×T_(s))long. Herein, T_(s) represents a sampling time and T_(s)=1/(15kHz×2048)=3.2552×10⁻⁸ (about 33 ns). A slot includes a plurality ofOrthogonal Frequency Division Multiplexing (OFDM) symbols or SC-FDMAsymbols in the time domain by a plurality of Resource Blocks (RBs) inthe frequency domain. In the LTE system, one RB includes 12 subcarriersby 7 (or 6) OFDM symbols. A unit time during which data is transmittedis defined as a Transmission Time Interval (TTI). The TTI may be definedin units of one or more subframes. The above-described radio framestructure is purely exemplary and thus the number of subframes in aradio frame, the number of slots in a subframe, or the number of OFDMsymbols in a slot may vary.

FIG. 5 illustrates exemplary control channels included in a controlregion of a subframe in a DL radio frame.

Referring to FIG. 5, a subframe includes 14 OFDM symbols. The first oneto three OFDM symbols of a subframe are used for a control region andthe other 13 to 11 OFDM symbols are used for a data region according toa subframe configuration. In FIG. 5, reference characters R1 to R4denote RSs or pilot signals for antenna 0 to antenna 3. RSs areallocated in a predetermined pattern in a subframe irrespective of thecontrol region and the data region. A control channel is allocated tonon-RS resources in the control region and a traffic channel is alsoallocated to non-RS resources in the data region. Control channelsallocated to the control region include a Physical Control FormatIndicator Channel (PCFICH), a Physical Hybrid-ARQ Indicator Channel(PHICH), a Physical Downlink Control Channel (PDCCH), etc.

The PCFICH is a physical control format indicator channel carryinginformation about the number of OFDM symbols used for PDCCHs in eachsubframe. The PCFICH is located in the first OFDM symbol of a subframeand configured with priority over the PHICH and the PDCCH. The PCFICHincludes 4 Resource Element Groups (REGs), each REG being distributed tothe control region based on a cell Identity (ID). One REG includes 4Resource Elements (REs). An RE is a minimum physical resource defined byone subcarrier by one OFDM symbol. The PCFICH is set to 1 to 3 or 2 to 4according to a bandwidth. The PCFICH is modulated in Quadrature PhaseShift Keying (QPSK).

The PHICH is a physical Hybrid-Automatic Repeat and request (HARQ)indicator channel carrying an HARQ ACK/NACK for a UL transmission. Thatis, the PHICH is a channel that delivers DL ACK/NACK information for ULHARQ. The PHICH includes one REG and is scrambled cell-specifically. AnACK/NACK is indicated in one bit and modulated in Binary Phase ShiftKeying (BPSK). The modulated ACK/NACK is spread with a Spreading Factor(SF) of 2 or 4. A plurality of PHICHs mapped to the same resources forma PHICH group. The number of PHICHs multiplexed into a PHICH group isdetermined according to the number of spreading codes. A PHICH (group)is repeated three times to obtain a diversity gain in the frequencydomain and/or the time domain.

The PDCCH is a physical DL control channel allocated to the first n OFDMsymbols of a subframe. Herein, n is 1 or a larger integer indicated bythe PCFICH. The PDCCH occupies one or more CCEs. The PDCCH carriesresource allocation information about transport channels, PCH andDL-SCH, a UL scheduling grant, and HARQ information to each UE or UEgroup. The PCH and the DL-SCH are transmitted on a PDSCH. Therefore, aneNB and a UE transmit and receive data usually on the PDSCH, except forspecific control information or specific service data.

Information indicating one or more UEs to receive PDSCH data andinformation indicating how the UEs are supposed to receive and decodethe PDSCH data are delivered on a PDCCH. For example, on the assumptionthat the Cyclic Redundancy Check (CRC) of a specific PDCCH is masked byRadio Network Temporary Identity (RNTI) “A” and information about datatransmitted in radio resources (e.g. at a frequency position) “B” basedon transport format information (e.g. a transport block size, amodulation scheme, coding information, etc.) “C” is transmitted in aspecific subframe, a UE within a cell monitors, that is, blind-decodes aPDCCH using its RNTI information in a search space. If one or more UEshave RNTI “A”, these UEs receive the PDCCH and receive a PDSCH indicatedby “B” and “C” based on information of the received PDCCH.

FIG. 6 illustrates a structure of a UL subframe in the LTE system.

Referring to FIG. 6, a UL subframe may be divided into a control regionand a data region. A Physical Uplink Control Channel (PUCCH) includingUplink Control Information (UCI) is allocated to the control region anda Physical uplink Shared Channel (PUSCH) including user data isallocated to the data region. The middle of the subframe is allocated tothe PUSCH, while both sides of the data region in the frequency domainare allocated to the PUCCH. Control information transmitted on the PUCCHmay include an HARQ ACK/NACK, a CQI representing a downlink channelstate, an RI for Multiple Input Multiple Output (MIMO), a SchedulingRequest (SR) requesting UL resource allocation. A PUCCH for one UEoccupies one RB in each slot of a subframe. That is, the two RBsallocated to the PUCCH are frequency-hopped over the slot boundary ofthe subframe. Particularly, PUCCHs with m=0, m=1, and m=2 are allocatedto a subframe in FIG. 6.

FIG. 7 is a diagram showing the structure of a general multiple inputmultiple output (MIMO) communication system according to an embodimentof the present invention.

MIMO refers to a method using multiple transmit antennas and multiplereceive antennas to improve data transmission/reception efficiency.Namely, a plurality of antennas is used at a transmitter or a receiverof a wireless communication system so that capacity can be increased andperformance can be improved. MIMO may also be referred to asmulti-antenna in this disclosure.

MIMO technology does not depend on a single antenna path in order toreceive a whole message. Instead, MIMO technology completes data bycombining data fragments received via multiple antennas. The use of MIMOtechnology can increase data transmission rate within a cell area of aspecific size or extend system coverage at a specific data transmissionrate. MIMO technology can be widely used in mobile communicationterminals and relay nodes. MIMO technology can overcome a limitedtransmission capacity encountered with the conventional single-antennatechnology in mobile communication.

FIG. 7 illustrates the configuration of a typical MIMO communicationsystem. A transmitter has N_(T) transmit (Tx) antennas and a receiverhas N_(R) receive (Rx) antennas. Use of a plurality of antennas at boththe transmitter and the receiver increases a theoretical channeltransmission capacity, compared to the use of a plurality of antennas atonly one of the transmitter and the receiver. Channel transmissioncapacity increases in proportion to the number of antennas. Therefore,transmission rate and frequency efficiency are increased. Given amaximum transmission rate R_(o) that may be achieved with a singleantenna, the transmission rate may be increased, in theory, to theproduct of R_(o) and a transmission rate increase rate R_(i) in the caseof multiple antennas, as indicated by Equation 1. R_(i) is the smallerof N_(T) and N_(R).R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For example, a MIMO communication system with four Tx antennas and fourRx antennas may theoretically achieve a transmission rate four timesthat of a single antenna system. Since the theoretical capacity increaseof the MIMO wireless communication system was verified in the mid-1990s,many techniques have been actively developed to increase datatransmission rate in real implementations. Some of these techniques havealready been reflected in various wireless communication standardsincluding standards for 3rd generation (3G) mobile communications,next-generation wireless local area networks, etc.

Active research up to now related to MIMO technology has focused upon anumber of different aspects, including research into information theoryrelated to MIMO communication capacity calculation in various channelenvironments and in multiple access environments, research into wirelesschannel measurement and model derivation of MIMO systems, and researchinto space-time signal processing technologies for improvingtransmission reliability and transmission rate.

Communication in a MIMO system will be described in detail throughmathematical modeling. It is assumed that N_(T) Tx antennas and N_(R) Rxantennas are present as illustrated in FIG. 7. Regarding a transmissionsignal, up to N_(T) pieces of information can be transmitted through theN_(T) Tx antennas, as expressed as the vector of Equation 2 below.s=└s ₁ ,s ₂ , . . . ,s _(N) _(T) ┘^(T)  [Equation 2]

Individual pieces of the transmission information s₁, s₂, . . . , s_(N)_(T) may have different transmit powers. If the individual transmitpowers are denoted by P₁, P₂, . . . , P_(N) _(T) , respectively, thenthe transmission power-controlled transmission information may be givenas shown in Equation 3.ŝ=[ŝ ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ]^(T)=[P ₁ s ₁ ,P ₂ s ₂ , . . . ,P _(N)_(T) s _(N) _(T) ]^(T)  [Equation 3]

The transmission power-controlled transmission information vector ŝ maybe expressed as shown in Equation 4 below, using a diagonal matrix P oftransmission power.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {{Ps}.}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

Meanwhile, NT transmission signals x₁, x₂, . . . , x_(N) _(T) to beactually transmitted may be configured by multiplying the transmissionpower-controlled information vector ŝ by a weight matrix W. The weightmatrix functions to appropriately distribute the transmissioninformation to individual antennas according to transmission channelstates, etc. The transmission signals x₁, x₂, . . . , x_(N) _(T) arerepresented as a vector X, as shown in Equation 5 below. Here, w_(ij)denotes a weight of an i-th Tx antenna and a j-th piece of information.W is referred to as a weight matrix or a precoding matrix.

$\begin{matrix}{x = {\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix} = {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{21} & w_{22} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}\hat{S_{1}} \\\hat{S_{2}} \\\vdots \\\hat{S_{j}} \\\vdots \\{\hat{S}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {WPs}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Generally, the physical meaning of the rank of a channel matrix is themaximum number of different pieces of information that can betransmitted on a given channel. Therefore, the rank of a channel matrixis defined as the smaller of the number of independent rows and thenumber of independent columns in the channel matrix. Accordingly, therank of the channel matrix is not larger than the number of rows orcolumns of the channel matrix. The rank of the channel matrix H(rank(H)) is restricted as shown in Equation 6 below.rank(H)≤min(N _(T) ,N _(R))  [Equation 6]

A different piece of information transmitted in MIMO is referred to as atransmission stream or stream. A stream may also be called a layer. Itis thus concluded that the number of transmission streams is not largerthan the rank of channels, i.e. the maximum number of different piecesof transmittable information. Thus, the channel matrix H is expressed asshown in Equation 7 below.# of streams≤rank(H)≤min(N _(T) ,N _(R))  [Equation 7]

“# of streams” denotes the number of streams. It should be noted thatone stream may be transmitted through one or more antennas.

One or more streams may be mapped to a plurality of antennas in manyways. This method may be described as follows depending on MIMO schemes.If one stream is transmitted through a plurality of antennas, this maybe regarded as spatial diversity. When a plurality of streams istransmitted through a plurality of antennas, this may be spatialmultiplexing. A hybrid scheme of spatial diversity and spatialmultiplexing may be contemplated.

It is expected that the future-generation mobile communication standard,LTE-A will support Coordinated Multi-Point (CoMP) transmission in orderto increase data rate, compared to the legacy LTE standard. CoMP refersto transmission of data to a UE through cooperation from two or moreeNBs or cells in order to increase communication performance between aUE located in a shadowing area and an eNB (a cell or sector).

CoMP transmission schemes may be classified into CoMP-Joint Processing(CoMP-JP) called cooperative MIMO characterized by data sharing, andCoMP-Coordinated Scheduling/Beamforming (CoMP-CS/CB).

In DL CoMP-JP, a UE may instantaneously receive data simultaneously fromeNBs that perform CoMP transmission and may combine the receivedsignals, thereby increasing reception performance (Joint Transmission(JT)). In addition, one of the eNBs participating in the CoMPtransmission may transmit data to the UE at a specific time point(Dynamic Point Selection (DPS)).

In contrast, in downlink CoMP-CS/CB, a UE may receive datainstantaneously from one eNB, that is, a serving eNB by beamforming.

In UL CoMP-JP, eNBs may receive a PUSCH signal from a UE at the sametime (Joint Reception (JR)). In contrast, in UL CoMP-CS/CB, only one eNBreceives a PUSCH from a UE. Herein, cooperative cells (or eNBs) may makea decision as to whether to use CoMP-CS/CB.

Hereinbelow, a description of channel state information (CSI) reportingwill be given. In the current LTE standard, a MIMO transmission schemeis categorized into open-loop MIMO operated without CSI and closed-loopMIMO operated based on CSI. Especially, according to the closed-loopMIMO system, each of the eNB and the UE may be able to performbeamforming based on CSI in order to obtain multiplexing gain of MIMOantennas. To acquire CSI from the UE, the eNB transmits RSs to the UEand commands the UE to feed back CSI measured based on the RSs through aPUCCH or a PUSCH.

CSI is divided into three types of information: an RI, a PMI, and a CQI.First, RI is information on a channel rank as described above andindicates the number of streams that can be received via the sametime-frequency resource. Since RI is determined by long-term fading of achannel, it may be generally fed back at a cycle longer than that of PMIor CQI.

Second, PMI is a value reflecting a spatial characteristic of a channeland indicates a precoding matrix index of the eNB preferred by the UEbased on a metric of signal-to-interference plus noise ratio (SINR).Lastly, CQI is information indicating the strength of a channel andindicates a reception SINR obtainable when the eNB uses PMI.

An advanced system such as an LTE-A system considers additionalmulti-user diversity through multi-user MIMO (MU-MIMO). Due tointerference between UEs multiplexed in an antenna domain in MU-MIMO,the accuracy of CSI may significantly affect interference with othermultiplexed UEs as well as a UE that reports the CSI. Accordingly, moreaccurate CSI than in single-user MIMO (SU-MIMO) should be reported inMU-MIMO.

In this context, the LTE-A standard has determined to separately designa final PMI as a long-term and/or wideband PMI, W1, and a short-termand/or subband PMI, W2.

For example, a long-term covariance matrix of channels expressed asEquation 8 may be used for hierarchical codebook transformation thatconfigures one final PMI with W1 and W2.W=norm(W1W2)  [Equation 8]

In Equation 1, W2 is a short-term PMI, which is a codeword of a codebookreflecting short-term channel information, W is a codeword of a finalcodebook, and norm(A) is a matrix obtained by normalizing each column ofmatrix A to 1.

Conventionally, the codewords W1 and W2 are given as Equation 9.

$\begin{matrix}{\mspace{79mu}{{{W\; 1(t)} = \begin{bmatrix}X_{i} & 0 \\0 & X_{i}\end{bmatrix}},{{{where}\mspace{14mu} X_{i}\mspace{14mu}{is}\mspace{14mu}{Nt}\text{/}2\mspace{14mu}{by}\mspace{14mu} M\mspace{14mu}{{matrix}.W}\; 2(j)} = {\overset{\overset{r\mspace{14mu}{columns}}{︷}}{\begin{bmatrix}e_{M}^{k} & e_{M}^{l} & \; & e_{M}^{m} \\{\alpha_{j}e_{M}^{k}} & {\beta_{j}e_{M}^{l}} & \ldots & {\gamma_{j}e_{M}^{m}}\end{bmatrix}}\mspace{14mu}\left( {{{if}\mspace{14mu}{rank}} = r} \right)}},{{{where}\mspace{14mu} 1}\mspace{14mu} \leq k},l,{m \leq {M\mspace{14mu}{and}\mspace{14mu} k}},l,{m\mspace{14mu}{are}\mspace{14mu}{{integer}.}}}} & \left\lbrack {{Equation}\mspace{14mu} 9} \right\rbrack\end{matrix}$

In Equation 9, the codewords are designed so as to reflect correlationcharacteristics between established channels, if cross-polarizedantennas are densely arranged, for example, the distance betweenadjacent antennas is equal to or less than half a signal wavelength. Thecross-polarized antennas may be divided into a horizontal antenna groupand a vertical antenna group and the two antenna groups are co-located,each having the property of a uniform linear array (ULA) antenna.

Therefore, the correlations between antennas in each group have the samelinear phase increment property and the correlation between the antennagroups is characterized by phase rotation. Since a codebook is quantizedvalues of channels, it is necessary to design a codebook reflectingchannel characteristics. For convenience of description, a rank-1codeword designed in the above manner may be given as Equation 10.

In Equation 9, the codebook configurations are designed to reflectchannel correlation properties generated when cross polarized antennasare used and when a space between antennas is dense, for example, when adistance between adjacent antennas is less than a half of signalwavelength. The cross polarized antennas may be categorized into ahorizontal antenna group and a vertical antenna group. Each antennagroup has the characteristic of a Uniform Linear Array (ULA) antenna andthe two groups are co-located.

Accordingly, a correlation between antennas of each group hascharacteristics of the same linear phase increment and a correlationbetween antenna groups has characteristics of phase rotation.Consequently, since a codebook is a value obtained by quantizing achannel, it is necessary to design a codebook such that characteristicsof a channel are reflected. For convenience of description, a rank-1codeword generated by the aforementioned configurations is shown inEquation 10 below.

$\begin{matrix}{{W\; 1(i)*W\; 2(j)} = \begin{bmatrix}{X_{i}(k)} \\{\alpha_{j}{X_{i}(k)}}\end{bmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

In Equation 10, a codeword is expressed as an N_(T)×1 vector where NT isthe number of Tx antennas and the codeword is composed of an uppervector X_(i)(k) and a lower vector α_(j)X_(i)(k), representing thecorrelation characteristics of the horizontal and vertical antennagroups, respectively. X_(i)(k) is expressed as a vector having thelinear phase increment property, reflecting the correlationcharacteristics between antennas in each antenna group. As arepresentative example, a discrete Fourier transform (DFT) matrix may beused.

An advanced system such as an LTE-A system considers achievement of anadditional multi-user diversity by the use of MU-MIMO. Due to theexistence of interference channels between UEs multiplexed in an antennadomain in MU-MIMO, the accuracy of CSI may significantly affectinterference with other multiplexed UEs as well as a UE that reports theCSI. Accordingly, more accurate CSI than in SU-MIMO should be reportedin MU-MIMO.

In CoMP JT, because a plurality of eNBs transmits the same data to aspecific UE through cooperation, the eNBs may be theoretically regardedas forming a MIMO system with antennas distributed geographically. Thatis, even when MU-MIMO is implemented in JT, highly accurate CSI isrequired to avoid interference between CoMP-scheduled UEs as in a singlecell MU-MIMO operation. The same applies to CoMP CB. That is, to avoidinterference with a serving cell caused by a neighbor cell, accurate CSIis needed. In general, a UE needs to report an additional CSI feedbackin order to increase the accuracy of CSI feedback. The CSI feedback istransmitted on a PUCCH or a PUSCH to an eNB.

Hereinafter, a vehicular communication system based on theabove-described wireless communication system will be described.

FIG. 8 is a diagram showing a vehicle including a plurality of antennaarrays according to an embodiment of the present invention. The numberof uses of the above-described wireless communication system and servicecategories using the wireless communication system have increased. Atthis time, unlike an existing static service, need to provide high datathroughput, high data rate and high quality of service (QoS) to userequipments (UEs) or users moving at a high speed has increased.

For example, a wireless communication system needs to support wirelessservices having good quality to moving UEs when a plurality of UEs orusers (hereinafter, collectively referred to as a UE) desires to viewmultimedia content while using public transportation or when a pluralityof UEs of passengers riding in a personal vehicle traveling on anexpressway uses different wireless communication services.

However, a conventional wireless communication system has some limits inprovision of a service to a UE in consideration of high-speed movementor mobility. At this time, in order to support a service, a systemnetwork needs to be revolutionized. In addition, a new system formaintaining compatibility with an existing network infrastructurewithout influencing the existing network infrastructure needs to bedesigned.

At this time, for example, as described below, a large-sized antennaarray may be mounted in a vehicle such that the vehicle acquires largearray gain, thereby providing services having good quality to UEslocated in the vehicle even in a state in which the vehicle moves at ahigh speed. At this time, in the vehicle, data received through acentral unit (hereinafter, CU) may be relayed to the UEs located in thevehicle. At this time, a vehicular MIMO system may be considered. Atthis time, as described above, if a large-sized antenna array is used,the vehicle can prevent communication performance from being lowered dueto penetration loss having an average value of 20 dB. In addition, sincethe vehicle uses receive (rx) antennas greater in number than the numberof UEs using a system, large array gain can be easily obtained andreception diversity can be obtained by ensuring a distance between thereceive antennas. That is, it is possible to provide a service to a UEmoving at a high speed without additionally designing a network throughthe vehicular MIMO system.

In spite of the above-described advantages, up to now, it has beendifficult to apply a vehicular MIMO system due to problems related tovehicle appearance and manufacturing system establishment. In addition,the vehicle is significantly expensive as compared to a personalportable communication device and cannot be easily improved and updated.In addition, since the vehicle should satisfy many requirements such asdesign concept and aeromechanical structure in addition to communicationperformance, the design of the vehicle may beaesthetically/aeromechanically restricted. For example, some vehiclemanufacturers have used complex antennas having quality inferior to thatof a single antenna in order to eliminate visual inconvenience of anexisting antenna.

In order to solve spatial restriction of a large-sized antenna array inan environment in which a communication system needs to be developed,installation of a distributed antenna array system for implementing amulti-antenna-array system in a vehicle has been gradually introduced inconsideration of vehicle appearance.

At this time, for example, referring to FIG. 8, a vehicle may include aplurality of antennas 810, 820, 830, 840, 850 and 860 mounted therein.At this time, the locations and number of the plurality of antennas 810,820, 830, 840, 850 and 860 may be changed according to vehicle design.At this time, the below-described configuration is equally applicableeven when the locations and number of the plurality of antennas 810,820, 830, 840, 850 and 860 mounted in the vehicle are changed, and thepresent invention is not limited to the below-described embodiments.That is, the present invention is applicable to antennas having variousshapes and radiation patterns according to the locations of theplurality of antennas 810, 820, 830, 840, 850 and 860.

At this time, signals for distributed antenna units (DUs) of the vehiclemay be controlled through a central unit (CU) 870. That is, the CU 870of the vehicle may control the signals for the DUs 810, 820, 830, 840,850 and 860 mounted in the vehicle to receive a signal from a basestation while maximizing reception diversity and to prevent wirelessconnection between the base station and the vehicle in a state in whichthe vehicle moves at a high speed. That is, the vehicle may be a UEhaving a plurality of antennas or a relay for relaying a signal. Thevehicle may provide a service having good quality to a plurality of UEslocated in the vehicle through control and relay of the signal receivedthrough the CU 870.

FIG. 9 is a diagram showing a method of selecting a distributed antennaunit (DU) in a state in which a plurality of vehicles is concentrated,according to an embodiment of the present invention.

As described above, a vehicle may include a plurality of DUs and a CU870 for controlling the DUs. At this time, a plurality of vehicles920-1, 920-2 and 920-3 may be concentrated in a narrow area. Forexample, the plurality of vehicles 920-1, 920-2 and 920-3 may beconcentrated in a narrow area upon city driving or upon a traffic jam.At this time, if the plurality of vehicles 920-1, 920-2 and 920-3 isconcentrated, it may be difficult to distinguish between beams for theDUs of the vehicles due to beam sharpness. For example, if a pluralityof vehicles is close to each other, the DU located at the right side ofthe first vehicle 920-1 may be adjacent to the DU located at the leftside of the second vehicle 920-2 and thus the beams for these DUs maynot be easily distinguished. That is, since DUs located adjacent to eachother receive signals undergoing similar channel environments, aplurality of DUs may be likely to receive the same beam or not toreceive a signal due to blocking of obstacles.

Accordingly, activation of the DUs deployed in the plurality of vehicles920-1, 920-2 and 920-3 needs to be controlled. More specifically, thevehicles 920-1, 920-2 and 920-3 may selectively control activation ordeactivation of the DUs based on the density of neighboring vehicles.For example, when a beam transmitted from a first base station 910-1 toa first vehicle 920-1 is received, the first vehicle 920-1 may activateonly the DUs located at the left side of the first vehicle 920-1 anddeactivate the remaining DUs of the first vehicle 920-1, to bedistinguished from the adjacent second vehicle 920-2. At this time, forexample, the first vehicle 920-1 may determine whether vehicles areconcentrated using a position information reception unit (e.g., a GPS)or a proximity sensor. In addition, for example, whether the DUs aredeactivated may be determined based on a threshold value based ondensity of vehicles. At this time, a threshold value may be a criterionvalue for determining activation or deactivation. That is, a criterionfor determining whether the vehicles 920-1, 920-2 and 920-3 areconcentrated may be changed and is not limited to the above-describedembodiment.

In addition, the third vehicle 920-3 may activate two DUs located at thefront side of the third vehicle 920-3 in order to receive the beam fromthe second base station 910-2. That is, the vehicles 920-1, 920-2 and920-3 may selectively activate/deactivate the DUs thereof to distinguishthe beam received through the activated DUs thereof from the beamscapable of being received by adjacent vehicles. Therefore, beams passingthrough independent paths experiencing different clusters are received,thereby improving beam reception performance.

In addition, the vehicles may feed information on activation anddeactivation of the DUs back to the base station as described above. Atthis time, for example, the above-described information may be fed backalong with channel state information (CSI) fed back from the vehicles tothe base station.

More specifically, a transmission end needs to obtain information on achannel and to accurately measure a suitable beam and gain obtained uponusing the beam based on the information. At this time, in a wirelesscommunication system, a reception end (e.g., a UE or a vehicle) may feedchannel information back to the transmission end (e.g., the basestation) in the form of CSI after measuring the channel.

At this time, for example, in a MIMO system, since a plurality ofantennas is used, a plurality of channels may be present and CSI may bedefined as a combination of sub-channels. At this time, as the number ofantennas used in the MIMO system increases, a complicated format may beused. In consideration of such an environment, an implicit CSI reportingscheme or an explicit CSI reporting scheme may be used as a CSIreporting scheme. That is, an implicit CSI reporting scheme or anexplicit CSI reporting scheme may be used as a CSI reporting scheme of amassive MIMO environment.

At this time, for example, the implicit CSI reporting scheme may referto a scheme of analyzing information on a channel measured by areception end and reporting only information substantially necessary togenerate a beam, without reporting the information on the channelmeasured by the reception end. That is, only necessary information maybe fed back based on a predefined or predetermined value.

In contrast, the explicit CSI reporting scheme may refer to a scheme ofreporting information maximally approximating to a measured value to atransmission end without a process of analyzing a channel measured by areception end. At this time, a method of quantizing a MIMO channelrepresented in a matrix or performing SVD operation may be used in thechannel information. For example, the implicit CSI report informationmay include a precoding matrix index (PMI), a channel quality indicator(CQI), rank information (RI), etc. In addition, the explicit CSI reportinformation may include channel coefficient quantization & quantizationindex feedback, MIMO matrix or vector quantization & quantization indexfeedback, channel covariance matrix feedback, Eigen matrix feedback(transmission of Eigen vectors and/or Eigen values of channel matrix),etc. At this time, the implicit CSI reporting scheme can reduce signaloverhead as compared to the explicit CSI reporting scheme, since onlynecessary information is extracted and fed back.

At this time, in association with the CSI feedback method of aconventional wireless communication system, a UE receives a pilot signal(reference signal) for channel estimation from a base station andcalculates and reports channel state information (CSI) to the basestation. At this time, the base station transmits data to the UE basedon the CSI fed back from the UE. At this time, in a wirelesscommunication system, the CSI fed back by the UE may include a channelquality indicator (CQI), a precoding matrix index (PMI), a rankindicator (RI), etc.

At this time, CQI feedback may be radio channel quality informationprovided to the base station for the purpose of providing informationregarding which modulation and coding scheme (MCS) is applied when thebase station transmits data (for link adaptation). When radio qualitybetween the base station and the UE is high, the UE may feed back a highCQI value and the base station may apply a relatively high modulationorder and a low channel coding rate and transmit data. Otherwise, the UEmay feed back a low CQI value and the base station may apply arelatively low modulation order and a high channel coding rate andtransmit data.

In addition, PMI feedback may be feedback of preferred precoding matrixinformation provided to the base station for the purpose of providinginformation regarding which MIMO precoding is applied when the basestation includes multiple antennas mounted therein. The UE may estimatea downlink MIMO channel between the base station and the UE from a pilotsignal and provide information indicating which MIMO precoding isapplied to the base station through PMI feedback. In a conventionalwireless communication system, only linear MIMO precoding representablein a matrix in a configuration of a PMI was considered. At this time,the base station and the UE share a codebook composed of a plurality ofprecoding matrices and each MIMO precoding matrix in the codebook has aunique index. Accordingly, the UE may feed back an index correspondingto a most preferred MIMO precoding matrix in the codebook as PMI,thereby minimizing the amount of feedback information of the UE.

Lastly, RI feedback may be feedback of information on the number ofpreferred transport layers provided to the base station for the purposeof providing information on the number of transport layers preferred bythe UE when each of the base station and the UE includes multipleantennas mounted therein and thus multilayer transmission throughspatial multiplexing is possible. At this time, since the base stationshould know which precoding is applied to each layer according to thenumber of transport layers, the RI may be closely related with the PMI.For example, in configuration of PMI/RI feedback, a PMI codebook may beconfigured based on single-layer transmission and then a PMI may bedefined and fed back per layer. However, in such a method, the amount ofPMI/RI feedback information is significantly increased as the number oftransport layers increases. Accordingly, in a conventional wirelesscommunication system, a PMI codebook according to the number oftransport layers was defined. That is, N matrices having a size of Nt×Rmay be defined in the codebook, for R-layer transmission (here, Rdenotes the number of layers, Nt denotes the number of transmit antennaports, and N denotes the size of a codebook). At this time, the size ofthe codebook may be defined regardless of the number of transportlayers. As a result, when the PMI/RI is defined in such a structure, thenumber R of transport layers becomes equal to the rank value of theprecoding matrix (Nt×R matrix) and thus may be referred to as a rankindicator (RI).

In addition, in a conventional wireless communication system, CSI may beobtained in an overall system frequency region or some frequency regions(e.g., Wideband CSI, Subband CSI). Particularly, in a system usingorthogonal frequency division multiple access (OFDMA) technology, CSI ofsome frequency regions (e.g., subband) preferred per UE may be obtainedand fed back.

At this time, the below-described PMI/RI may not be limited to the indexvalue of a precoding matrix represented in an Nt×R matrix and the rankvalue of a precoding matrix like a PMI/RI of a wireless communicationsystem. In addition, the below-described PMI indicates preferred MIMOprecoder information among MIMO precoders applicable to a transmissionend and the precoder is not limited to a linear precoder represented ina matrix as in a conventional wireless system. In addition, thebelow-described RI has a broader meaning than the RI in the conventionalwireless communication system and may include all feedback informationindicating the number of preferred transport layers without beinglimited thereto.

In addition, for example, the PMI value may not include only one index.For example, in the conventional wireless communication system, a finalPMI is divided into W1 which is a long term and/or wideband (WB) PMI andW2 which is a short term and/or subband (SB) PMI, thereby designing aPMI having a dual structure. At this time, when the final PMI is W,W=W1*W2 or W=W2*W1 may be defined. In addition, for example, in an LTE-Asystem, if the number of transmit antenna ports is 8 or if the number oftransmit antenna ports is 4 andalternativeCodeBookEnabledFor4TX−r12=TRUE is configured through RRCsignaling, a final MIMO precoding matrix may be derived by onlycombining two indices (WB PMI & SB PMI).

In addition, in a wireless communication system, in single user-MIMO(SU-MIMO), only data of one UE may be scheduled in the sametime/frequency domain. That is, if information is transmitted to andreceived from one UE by MIMO, only scheduling information of one UE maybe included in one time/frequency domain. In contrast, in multiuser-MIMO(MU-MIMO), data of a plurality of UEs may be scheduled together in onetime/frequency domain. At this time, in MU-MIMO, the data is multiplexedin the same time/frequency domain, thereby obtaining additional gain.However, if the plurality of UEs is scheduled together, co-channelinterference is generated by the UEs, thereby deteriorating systemperformance. At this time, the UE may feed CSI thereof back to the basestation and the base station may schedule a user based on the CSI fedback from the plurality of UEs, thereby optimizing a system.

However, if a new UE is further scheduled in an SU-MIMO state or anMU-MIMO state, in a conventional wireless communication system,influence of interference between users generated by scheduling the newUE in the system may not be considered. That is, since only channelinformation considering SU-MIMO is fed back and the base station onlychecks the channel state of each user and cannot acquire information oninterference to be experienced by each user in MU-MIMO, it may bedifficult to reduce influence of interference between UEs. Accordingly,when SU-MIMO is switched to MU-MIMO or if MU-MIMO operates, multiplexinggain capable of being obtained by supporting multiple UEs needs to besufficiently considered.

At this time, in consideration of the above-described situation, if aplurality of vehicles (or UEs) (hereinafter, collectively referred to asvehicles) is present, each vehicle may feed combination informationrelated to the number and locations of DUs activated in the vehicle andchannel state information back to a base station. That is, interferencewith other vehicles and reception performance may be changed accordingto the number and locations of DUs activated in each vehicle and eachvehicle needs to feed information on the activated DUs back to the basestation. At this time, the base station may acquire an effective channelof each vehicle based on the received channel state information and theactivated DU information and transmit data. At this time, as describedabove, in a state in which vehicles are concentrated, the base stationneeds to acquire more accurate channel information. Hereinafter, amethod of feeding back activated DU information at a vehicle will bedescribed. Although a vehicle including a DU will be described below,the method is equally applicable to a UE including a plurality ofantennas or another device including a plurality of antennas andoperating based on the plurality of antennas and is not limited to thebelow-described embodiment.

FIG. 10 is a diagram showing an example of a DU selection combinationaccording to an embodiment of the present invention. As described above,an implicit CSI reporting scheme or an explicit CSI reporting scheme maybe used as a CSI reporting scheme. At this time, if a plurality of DUsis included in a vehicle and activation of DUs is selectable, effectivechannel information may be changed according to the number and locationsof activated DUs.

More specifically, in a conventional communication system, if aplurality of antennas is present and all antennas are activated, a UEmay feed back CSI. However, as described above, if a plurality ofvehicles is concentrated or if a plurality of UEs or devices isconcentrated, cooperative communication may be performed inconsideration of mutual interference. At this time, each UE may selectDUs to be activated from among the plurality of DUs included therein,thereby reducing interference with the other vehicles or UEs. At thistime, channel information to be reported by the vehicle may be changedaccording to the number and locations of activated DUs among theplurality of DUs and thus a channel information reporting methodconsidering the same may be necessary.

At this time, for example, the channel information reported by thevehicle may include channel state information and DU index setinformation combined according to the number and locations of activatedDUs among the plurality of DUs included in the vehicle. At this time,the channel state information may be explicitly reported inconsideration of the DU index set information. At this time, forexample, the explicitly reported channel state information may includeat least one of the channel coefficient, quantization & quantizationindex feedback, MIMO matrix or vector quantization & quantization indexfeedback, channel covariance matrix feedback and Eigen matrix feedback(transmission of Eigen vectors and/or Eigen values of channel matrix).In addition, for example, the channel information fed back by thevehicle may include at least one of the DU index set information and thechannel state information. That is, the channel information fed back bythe vehicle may include both or one of the DU index set information andthe channel state information, without being limited to theabove-described embodiment.

In addition, the channel information fed back by the vehicle may includeexplicit channel state information in consideration of the DU index setas described above, without being limited thereto. For example, thevehicle may report the DU index set information and implicit channelstate information to the base station and the base station may acquirefinal effective channel information using the implicit channel stateinformation and the DU index set information.

In addition, the base station may receive the channel information ofeach of the plurality of vehicles. That is, the base station may receivethe DU index set information and the channel state information fed backby each of the plurality of vehicles. At this time, the base station mayacquire effective channel information using the received DU index setinformation and channel state information. The base station may transmitdata to the vehicles using the acquired effective channel information.

At this time, the DU index set information may differ between thevehicles. As described above, the DU index set information may bedifferently set based on the number and locations of activated DUs amongthe plurality of DUs included in the vehicle.

For example, referring to FIG. 10(a), the vehicle may include four DUs1010, 1020, 1030 and 1040 at corner regions thereof. At this time, thenumber and locations of DUs included in the vehicle may be differentlyset and is not limited to the above-described embodiment. In addition,for example, each vehicle may feed the number and locations of DUsincluded therein back to the base station. In addition, for example,each vehicle may feed information on the number and locations ofactivated DUs among the DUs included therein back to the base station.In addition, for example, the base station may acquire the DUinformation of each vehicle through higher layer signaling or anotherpath, without being limited to the above-described embodiment.

At this time, for example, FIG. 10(a) shows the case where only one ofthe four DUs 1010, 1020, 1030 and 1040 included in the vehicle isactivated. At this time, different combinations may be configuredaccording to selected DUs. More specifically, whether each of the DUs1010, 1020, 1030 and 1040 included in the vehicle is activated may beindicated through a matrix or an index. At this time, if only the DUlocated at the front left side of the vehicle is activated, the DU indexmay be [1 0 0 0]. If only one DU included in the vehicle is activatedusing the same method, the DU index may be represented by [0 1 0 0], [00 1 0] or [0 0 0 1].

At this time, for example, the DU index set information may include theabove-described four pieces of DU index information. In addition, forexample, the vehicle may feed information indicating that only one ofthe four DUs 1010, 1020, 1030 and 1040 is activated back to the basestation. At this time, the vehicle may include the four pieces of DUindex information in the DU index set information and feed the DU indexset information back to the base station along with the channel stateinformation. At this time, the base station may acquire effectivechannel information based on the DU index information and performdownlink data transmission.

In addition, for example, the vehicle may feed the DU index setinformation considering both the number and locations of DUs includedtherein back to the base station. At this time, for example, the numberof cases of selecting M_(i)(M_(i)≤N_(i)) DUs to be activated among theDUs included in one vehicle is shown in Equation 11 below.N _(i) ^(C) M _(i)  [Equation 11]

Accordingly, the total number of cases of DU combinations selectablefrom one vehicle is shown in Equation 12 below.

$\begin{matrix}{{\sum\limits_{M_{i} = 0}^{N_{i}}\;{{}_{Ni}^{}{}_{Mi}^{}}} = 2^{N_{i}}} & \left\lbrack {{Equation}\mspace{14mu} 12} \right\rbrack\end{matrix}$

In addition, for example, if M_(i)=0, the vehicle may be deactivated notto cause interference with the other vehicles. At this time, forexample, the vehicle may feed the DU index set information including2^(N) ^(i) pieces of DU index information back to the base station. Thebase station may determine an effective channel using the fed-back DUindex set information and channel state information.

In addition, for example, FIG. 10(b) shows DU index information which isdifferently set according to the number and locations of activated DUsamong the four DUs 1010, 1020, 1030 and 1040 of the vehicle. That is,the DU index may be determined based on the number and locations ofactivated DUs among the DUs included in the vehicle and the DU indexinformation may be fed back to the base station as DU index setinformation.

At this time, for example, in FIG. 10(a), only one of the four DUs maybe activated in one vehicle. At this time, as described above, the DUindex set may be given as {[1 0 0 0], [0 1 0 0], [0 0 1 0], [0 0 0 1].At this time, the vehicle may transmit the channel state information tothe base station as an explicit CSI report. At this time, for example,the explicit CSI report may include MIMO matrix quantization. At thistime, MIMO matrix quantization is shown in Equation 13 below.

$\begin{matrix}{H = \begin{bmatrix}h_{1} \\h_{2} \\\vdots \\h_{N_{i}}\end{bmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 13} \right\rbrack\end{matrix}$

At this time, h_(i) elements configuring the matrix may be a scalarvalue by the number of antennas in each DU, quantization resolution,etc. In addition, for example, h_(i) may be a row vector or a columnvector. At this time, for example, if h_(i) is represented in the formof a column vector, a MIMO matrix may be represented by horizontallystacking h_(i), not vertically stacking h_(i) as in Equation 13. Thatis, the base station, which has received the explicitly reported CSI andthe selectable DU index set, may combine and use the effective channelinformation of the vehicle using the information for downlinktransmission. At this time, for example, in Equation 13, the effectivechannel information capable of being extracted from explicit feedbackand the DU index set [0 1 0 0] may become H_(eff)=[h₂]. That is, thebase station may extract some (rows or columns) of the explicit CSIbased on the DU index set information fed back from each vehicle, formeffective channel information of each vehicle and perform datatransmission using the effective channel information.

FIG. 11 is a diagram showing a DU including a plurality of antenna portsaccording to an embodiment of the present invention. As described above,each vehicle may include DU index set information and channel stateinformation in channel information and feed the channel information backto the base station.

At this time, for example, the DU index set may be configured as antennaport group (sub grouping or antenna port grouping) information per DU.

More specifically, referring to FIG. 11, one vehicle may include fourDUs composed of 2/2/4/4 physical antennas. At this time, the totalnumber of physical antennas included in each DU may be 12. At this time,for example, the vehicle may generate channel state information based onthe total number of physical antennas. At the same time, the vehicle mayfeed information indicating the antenna port of the DU, to which eachphysical antenna is mapped, back to the base station through the DUindex set information.

For example, the vehicle may include information such as DU1 (port#={0,1}, number of antennas=2), DU2 (port #={2,3}, number of antennas=2)in the DU index set and feed the information back to the base station.At this time, the base station may map the antenna port index using thereceived DU index set information and the channel state information andtransmit data while controlling interference between the vehicles.

FIG. 12 is a diagram showing a method of activating a DU based on an RFstructure according to an embodiment of the present invention. Inaddition, for example, each vehicle may transmit, to the base station,information on how many DUs are simultaneously activated/deactivated(on/off) by the vehicle based on the radio frequency (RF) structure. Atthis time, for example, the RF end may correspond to the below-describedtransmission or reception module, which will be described below.

At this time, for example, referring to FIG. 12, when a plurality of DUsis configured to be switched or is connected to one RF chain accordingto the structure of the RF end in each vehicle, the number of DUsactivated/deactivated simultaneously and a combination thereof may havesome limits.

At this time, the vehicle may feed the above-described information backto the base station along with the DU index set. At this time, the basestation may consider a combination of selectable DUs from each vehiclein advance based on the above-described information and select amultiuser precoder in consideration of interference with a plurality ofvehicles. At this time, in the above-described configuration, unlike auser in MU-MIMO of a conventional wireless communication system, DUs maybe deployed in one UE to control a direction of a received beam throughDU selection.

In addition, for example, each vehicle may differently set a method offeeding back channel information including DU index set information andchannel state information.

More specifically, each vehicle may feed channel information of eachvehicle back to the base station. In addition, for example, a first typeof vehicles among the vehicles may directly feed channel informationback to the base station. At this time, the base station may control DUselection of each vehicle using the DU index set information and thechannel state included in the channel information received from eachvehicle in consideration of interference between vehicles in a state inwhich a plurality of vehicles is present, and determine a precoder tocontrol interference.

In addition, for example, each vehicle may transmit channel informationthereof to all or some of the other vehicles. At this time, for example,a vehicle for transmitting channel information to another vehicle may bea second type of vehicle. At this time, for example, each vehicle maycalculate influence of interference using the channel informationthereof and the channel information received from the other vehicles.Therefore, each vehicle may perform DU selection and determine aprecoder.

That is, the base station may determine selection of activated DUs inconsideration of the number and locations of DUs included in the vehiclethrough the channel information of a plurality of vehicles and controlvehicular communication using a method of informing the vehicle of thedetermined information in an environment in which a plurality ofvehicles is concentrated. In addition, the plurality of vehicles maydirectly control DU selection by exchanging channel information to eachother, thereby controlling vehicular communication.

In addition, for example, a combination of the above-described methodsmay be performed in each vehicle. More specifically, all vehiclespresent in one system do not need to select one of the above-describedtwo methods and to operate using the same method.

For example, the first type of vehicles may transmit channel informationto the base station and perform DU selection under control of the basestation. In addition, the second type of vehicles may receive channelinformation from the other vehicles and directly control DU selection.

In addition, for example, specific vehicles of the plurality of vehiclesmay transmit channel information configured thereby to the othervehicles and vehicles which do not exchange channel information with theother vehicles may directly feed channel information thereof back to thebase station. In addition, some of the vehicles which exchange channelinformation configured thereby may feed channel information of all orsome of the vehicles that they know back to the base station.

Although the vehicle including the plurality of antennas is described inthe above-described configuration, the present invention is not limitedthereto and the same method is applicable to a UE or a device operatingin a general multi-user MIMO system.

FIG. 13 is a flowchart illustrating a method of transmitting andreceiving a signal according to an embodiment of the present invention.A vehicle (or a UE) may receive a reference signal (S1310). Thereafter,the vehicle may feed back channel information based on the receivedreference signal (S1320). Thereafter, the vehicle may receive data basedon the channel information (S1330). At this time, as described abovewith reference to FIGS. 8 to 12, the channel information may include DUindex set information and channel state information. At this time, forexample, as described above, the vehicle may include a plurality of DUs.At this time, the DU index set information may be determined based onthe number and locations of activated DUs of the DUs included in thevehicle. At this time, the base station may acquire effective channelinformation using the DU index set information and the channel stateinformation and perform downlink transmission. In addition, for example,the DU index set information fed back by the vehicle may further includemapping relationship information between the DUs and the antenna portsconfigured in the DUs. At this time, the base station may acquireinformation on the antenna ports using the DU index set information andthe channel state information, acquire effective channel information anddetermine a precoder.

In addition, for example, DU selection for each vehicle may bedetermined by the base station based on the channel informationtransmitted from the vehicle to the base station and signaled to thevehicle. At this time, for example, as described above, DU selection maymean information on the locations and number of activated DUs among theDUs included in the vehicle. In addition, for example, DU selection foreach vehicle may be directly determined by each vehicle. At this time,for example, each vehicle may acquire channel information fromneighboring vehicles and calculate influence of interference. Eachvehicle may perform DU selection in consideration of influence ofinterference and feed information on DU selection back to the basestation, thereby performing communication. In addition, a combination ofthe above-described two methods may be used without being limited to theabove-described embodiment.

FIG. 14 is a block diagram showing a UE apparatus and a base stationapparatus according to an embodiment of the present invention.

At this time, the base station apparatus 100 may include a transmissionmodule 110 for transmitting a radio frequency signal, a reception module130 for receiving a radio frequency signal, and a processor 120 forcontrolling the transmission module 110 and the reception module 130. Atthis time, the base station apparatus 100 may perform communication withan external device using the transmission module 110 and the receptionmodule 130. At this time, the external device may be a UE. That is, thebase station apparatus 100 may perform communication with the UEapparatus 100 as an external device, without being limited thereto.

In addition, the UE apparatus 100 may include a transmission module 210for transmitting a radio frequency signal, a reception module 230 forreceiving a radio frequency signal, and a processor 220 for controllingthe transmission module 210 and the reception module 230. At this time,the UE apparatus 200 may perform communication with the base stationusing the transmission module 210 and the reception module 230. That is,the UE apparatus 200 may perform communication with the base station ina wireless communication system, without being limited thereto. At thistime, for example, the transmission module 210 and the reception module220 may be the above-described RF ends. That is, the UE apparatus 200may switch or control activation of DUs using the transmission module210 and the reception module 220. More specifically, the UE apparatus200 may transmit a signal to an external device through the transmissionmodule 210 and the reception module 220 and thus the transmission module210 and the reception module 220 may correspond to DUs which are aplurality of antennas.

At this time, for example, the processor 220 of the UE apparatus 200 mayreceive a reference signal using the reception module 220 and feed backchannel information using the transmission module 210 based on thereceived reference signal. In addition, the processor 220 of the UEapparatus 200 may receive data using the reception module 220 based onthe channel information. At this time, if the UE apparatus 200 includesa plurality of DUs and activation of each of the plurality of DUs isselectable, the channel information may include DU index set informationand channel state information (CSI). At this time, for example, the DUindex set information may be determined according to the number ofactivated DUs and the locations of the DUs based on activation of eachof the plurality of DUs.

In addition, for example, the above-described UE apparatus 200 maycorrespond to the above-described vehicle. That is, one vehicle may beone UE apparatus 200 and each element may be included in the vehicle.

The embodiments according to the present invention can be implemented byvarious means, for example, hardware, firmware, software, orcombinations thereof.

In the case of a hardware configuration, the embodiments of the presentinvention may be implemented by one or more application specificintegrated circuits (ASICs), digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), processors, controllers,microcontrollers, microprocessors, etc.

In the case of a firmware or software configuration, the methodaccording to the embodiments of the present invention may be implementedby a module, a procedure, or a function, which performs functions oroperations described above. For example, software code may be stored ina memory unit and then may be executed by a processor. The memory unitmay be located inside or outside the processor to transmit and receivedata to and from the processor through various well-known means.

The detailed description of the exemplary embodiments of the presentinvention has been given to enable those skilled in the art to implementand practice the invention. Although the invention has been describedwith reference to the exemplary embodiments, those skilled in the artwill appreciate that various modifications and variations can be made inthe present invention without departing from the spirit or scope of theinvention described in the appended claims. Accordingly, the inventionshould not be limited to the specific embodiments described herein, butshould be accorded the broadest scope consistent with the principles andnovel features disclosed herein. Although the preferred embodiments ofthe present invention have been disclosed for illustrative purposes,those skilled in the art will appreciate that various modifications,additions and substitutions are possible, without departing from thescope and spirit of the invention as disclosed in the accompanyingclaims. Such modifications should not be individually understood fromthe technical spirit or prospect of the present invention.

Both apparatus and method inventions are mentioned in this specificationand descriptions of both of the apparatus and method inventions may becomplementarily applicable to each other.

INDUSTRIAL APPLICABILITY

Although an example of applying a method of transmitting and receiving asignal in a vehicular communication system and an apparatus therefor toa 3GPP LTE system is described, the present invention is applicable tovarious wireless communication systems in addition to the 3GPP LTEsystem.

The invention claimed is:
 1. A method for a user equipment (UE)including a plurality of distributed antenna units (DUs) to transmit andreceive a signal with a base station, the method comprising: receiving areference signal; transmitting information on the number of DUs andphysical locations of the DUs to the base station; transmitting channelinformation including DU index set information including a plurality ofDU indexes and channel state information (CSI) based on the receivedreference signal to the base station, receiving, from the base station,information on one DU index determined by the base station based on theDU index set information and the CSI; and receiving, from the basestation, data via one or more DUs activated according to the informationon the one DU index, wherein the plurality of DU indexes is determinedby the UE based on the number and the physical locations of at least oneDU that can be activated.
 2. The method according to claim 1, whereinthe DU index set information includes 2{circumflex over ( )}N pieces ofDU index information when the number of DUs included in the UE is N, andwherein N is integer.
 3. The method according to claim 1, wherein thedata is received by the base station based on the DU index setinformation included in the channel information and effective channelinformation acquired through the channel state information.
 4. Themethod according to claim 1, wherein each of the DUs included in the UEincludes a plurality of antenna ports.
 5. The method according to claim4, wherein the DU index set information includes information on amapping relationship between the plurality of antenna ports and theplurality of DUs.
 6. The method according to claim 1, further comprisingfeeding back DU activation information.
 7. The method according to claim6, wherein the DU activation information includes a number of DUscapable of being simultaneously activated and deactivated and the DUindex set based on a radio frequency (RF) structure.
 8. The methodaccording to claim 1, wherein, the UE receives the channel informationfrom neighboring UEs, and wherein the CSI further includes the channelinformation received from the neighboring UEs.
 9. A user equipment (UE)apparatus including a plurality of distributed antenna units (DUs) fortransmitting and receiving a signal, the UE comprising: a receiverconfigured to receive information from an external device; a transmitterconfigured to transmit information to the external device; and aprocessor configured to control the receiver and the transmitter,wherein the processor is further configured to: receive a referencesignal using the receiver, transmit information on the number of DUs andphysical locations of the DUs to the base station using the transmitter,transmit channel information including DU index set informationincluding a plurality of DU indexes and channel state information (CSI)based on the received reference signal to the base station using thetransmitter, wherein the DU index set information includes informationon DU index sets for a combination of DUs selectable for activationamong the plurality of DUs; receiving, via the receiver, information onone DU index set, determined by the base station based on the DU indexset information and the CSI from the base station, and receive data,using the receiver, based on one or more DUs activated according to theinformation on the one DU index from the base station, wherein theplurality of DU indexes is determined by the UE based on the number andthe physical locations of at least one DU that can be activated.